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A major new United Nations report shows global scientists are more convinced than ever that human activity is causing climate change, the Toronto Star has learned.

The rate of warming between now and 2030 is likely to be twice that of the previous century, it says.

And it concludes that most of the global warming since the middle of the last century has been caused by man-made greenhouse gases.

The report, to be released in Paris Feb. 2, should all but end any debate on climate change and compel governments and industries to take urgent measures to deal with it, scientists say.

"It is very likely that (man-made) greenhouse gas increases caused most of the globally average temperature increases since the mid-20th century," states the report of the Intergovernmental Panel on Climate Change.

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In the clinical language of science, it paints a stark picture of the effects of greenhouse gas emissions:

"Discernible human influences now extend to other aspects of climate, including continental average temperatures, atmospheric circulation patterns and some types of extremes."

It is "very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent." Storm tracks will move from the tropics toward the poles.

The widely anticipated report is the fourth by the IPCC, which every few years publishes the definitive conclusions of about 2,000 scientists who are recognized as experts in their respective fields. Each one has moved closer to closing debate on the causes and effects of climate change.

The portion of the report obtained by the Star is called the final draft of the "Summary for Policy Makers."

The summary states that the warming effect of greenhouse gases increased by 20 per cent during the past decade – "the largest change observed or inferred for any decade in at least the last 200 years."

Global warming would be even greater had it not been slowed by other forms of pollution that stopped some of the sun's energy from reaching the Earth.

Rebutting one of the main arguments of climate change skeptics, it says observations of temperature increases and shrinking ice cover, "support the conclusion that it is extremely unlikely that global climate change of the past 50 years" was caused by solar flares or other natural events.

Eleven of the past 12 years have been the hottest in Earth's recent history, it says.

All the continents except Antarctica have warmed during the past half-century, with the biggest impacts in Canada's Arctic and other northern regions.

Research since the third report was released in 2001 increases the certainty about climate change and the likely scale of most of its effects, including warmer temperatures and severe weather, the report states.

One crucial prediction has been made a bit less worrying: Although sea level is rising – for now, mainly because the oceans are warming to a depth of at least 3,000 metres, and expanding – the estimates for how much it will go up have been lowered.

The summary also notes that there has been, as yet, little change in the North Atlantic Drift, the warm current that gives Britain and northern Europe a relatively temperate climate and that is expected to slow, or stop, as climate change alters the ocean.

It will slow, but not abruptly during the coming century, the report says.

For the most part, though, the conclusions point in a single direction:

"Warming of the climate system is unequivocal."

The report estimates that if the amount of carbon dioxide in the atmosphere could be kept below 550 parts per million – which would take a major worldwide effort to cut greenhouse gas emissions – the average global temperature would rise by 2 to 4.5 degrees Celsius above the level before the Industrial Revolution started about 250 years ago.

The current carbon level is about 380 parts per million and rising steadily, compared with 280 at the time humans began burning large amounts of coal, oil and other fossil fuels.

The temperature estimate depends on which combination of computer model and research data is used.

The upper forecast is higher than in previous reports.

"Values higher than 4.5 C cannot be excluded" because of "feedbacks," such as the increased ability of the atmosphere to absorb water vapour – an extremely potent greenhouse gas – as it heats up, and the greater warmth absorbed as Arctic ice melts.

Regional forecasts of climate change effects are better than in the previous report, and they predict the greatest warming at northern latitudes and high altitudes, and the least over the North Atlantic and the southern oceans.

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